1. Field of the Invention
This invention relates to a light interferometer for detecting the direction of phase change of a measuring light, i.e., the direction toward which the optical path length of a measuring light is increased or decreased according to an interference signal which is changed in accordance with the change of the optical path of length of the measuring light. The invention is also relates to an optical integrated circuit for the use in the light interferometer.
2. Description of the Prior Art
Heretofore, there has been known a bulk type phase modulation interferometer as shown in FIG. 15. This conventional bulk type phase modulation interferometer includes a laser beam light source 1. The laser beam light source 1 emits a laser beam as a coherent light P. An outgoing optical path of the laser beam is provided at its midway with a half mirror 2 for dividing the optical path.
The half mirror 2 divides the laser beam into a reference light P.sub.1 and a measuring light P.sub.2. The reference light P.sub.1 is reflected by a reference prism 3, whereas the measuring light P.sub.2 is reflected by a measuring prism 4. The reference light P.sub.1 reflected by the reference prism 3 and the measuring light P.sub.2 reflected by the measuring prism 4 are interfered with each other and guided to a photodetector 5 as an interference light. In the bulk type phase modulation, the measuring prism 4 is moved in the direction as shown by an arrow or in the direction opposite thereto, whereas the reference prism 3 is periodically vibrated at a constant amplitude and at a predetermined cycle.
In this way, if the optical path length of the reference light P.sub.1 is periodically changed at a predetermined amplitude, an interference signal, which is changed in accordance with the change of the optical path length of the measuring light P.sub.2, is taken off the photodetector 5.
The vibrating frequency of the reference prism 3 is represented by f.sub.1, and an alternating current-like changing composition corresponding to the vibrating frequency f.sub.1 and an alternating current-like changing composition corresponding to frequency which is two times of the vibration frequency f.sub.1 are extracted out of the frequency compositions of the interference signal, respectively. Since the amplitude of the extracted alternating current-like changing composition becomes a sine-wave and a cosine-wave when the measuring prism 4 is moved, there can be obtained two kinds of electric signals which are different in phase difference by .pi./2 based on the extracted two kinds of different alternating current-like compositions. Therefore, if the two kinds of electric signals, which are different in phase difference by .pi./2, are processed, there can be found a phase change direction (the moving direction of the measuring prism 4) of the measuring light and there can also be measured the moving amount of the measuring prism 4 without being affected by a direct current bias composition based on the change of light quantity.
Therefore, if this light interferometer is used in a length measuring machine, the length of an objective substance can be measured in such degree of accuracy as less than a portion of the unit of wavelength. The moving direction of the objective substance can also be measured.
The alternating current-like changing composition corresponding to the vibrating frequency f.sub.1 and the alternating current-like changing composition corresponding to the frequency two times of the vibrating frequency f.sub.1 are extracted out of the frequency compositions of the interference signal by electric processing using a band pass filter comprising a CR circuit, etc.
Also, there has been known a recurrent optical system type light integrated interferometer as shown in FIGS. 16 and 17. In FIGS. 16 and 17, 6 denotes a thin film substrate formed with a two-dimension type wave guide passageway. The thin film substrate 6 comprises three layers of thin films 7, 8 and 9. The thin film 7 has at least a light permeability. The refractive index of the thin film 7 is larger than those of the thin films 8 and 9 at both sides thereof. The coherent light P emitted by a light source 10 is made incident to the thin film substrate 6. The coherent light P is reflected by two interfaces or boundary surfaces between the thin film 7 and the thin films 8, 9 and propagated through the interior of the thin film 7. The thin film substrate 6 is provided at its incidence side with a collimater lens system 11. The coherent light P made incident to the thin film 7 is made into a parallel pencil of rays by the collimater lens system 11. The parallel pencil of rays are divided into a reference light P.sub.1 and a measuring light P.sub.2.
The reference light P.sub.1 is reflected by a reference mirror 13 formed on the thin film substrate 6 and returned to the half mirror system 12 again. The measuring light P.sub.2 is reflected by a measuring mirror 14 as an object and returned to the half mirror system 12. The returned measuring light P.sub.2 and reference light P.sub.1 are composed by the half mirror system 12 and introduced to a measuring lens 15 as an interference light. The interference light is emitted outside the film from an outgoing prism 16. The interference light emitted by the prism 16 is dark under the conditions that the wavelength of the coherent light P is represented by .lambda., and the difference of the optical path length of the reference light P.sub.1 multiplies oddly as against .lambda./2. On the other hand, when the difference multiplies integrally as against .lambda./2, the interference light is bright. Therefore, if the measuring mirror 14 is moved in the direction as shown by an arrow G, the interference signal based on the interference light has a bright portion A and a dark portion B alternately, as shown in FIG. 18, every time the moving amount is increased by .lambda./2. Therefore, by counting the number of the bright and dark portions A and B, the moving amount of the measuring mirror 14 can be found. Similarly, by counting the moving amount from the origin, the length of the objective substance can be measured.
However, in the case of the bulk type phase modulation interferometer, since the reference prism 3 itself is large, the reference prism 3 is difficult to be vibrated stably at a constant vibration and at a constant cycle. Moreover, it is not preferable that the alternating current-like changing component corresponding to the vibrating frequency f.sub.1 and the alternating current-like changing composition corresponding to the frequency two times the vibrating frequency f.sub.1 are extracted out of the frequency compositions of the interference signal by electric processing using a band pass filter comprising a CR circuit, etc., because the frequency characteristics are unstable due to change of temperature, etc. On the other hand, in the case of the light integrated type interferometer, even if the measuring mirror 14 is moved in the reversed direction H, the bright and dark portions A and B are produced in the same manner. Accordingly, the phase changing direction of the measuring light cannot be detected. Furthermore, since the light intergrated type interferometer directly receives the fluctuation of the direct current bias composition, it is difficult to obtain such degree of accuracy as .lambda./2 or more.
Furthermore, in this kind of light integrated type interferometer, the measuring light P.sub.2 reflected by the mirror 14 is partly reflected by the half mirror system 12 and returned to the light source 10. Similarly, the reference light P.sub.1 reflected by the reference mirror 13 is also partly passed through the half mirror 12 and returned to the light source 10. Accordingly, the output of the light source 10 is fluctuated by the return light, and an accurate measurement is difficult to carry out.